Digital Microfluidics

Background

It has been realized in recent years that the classical concept of microfluidics, i.e., the confinement of the flow of single phase liquids to networks of narrow channels, has some fundamental drawbacks. The most obvious one is the fact that in small dimensions, the Reynolds number Re of the flow rarely exceeds unity, such that the flow is purely laminar (the threshold for turbulence is Re > 200). As a consequence, mixing two substances is difficult, and may proceed only by diffusion or, in some cases, be achieved via viscous dephasing. Hence, there has been considerable development towards the use of isolated water droplets suspended in an oily phase. The main advantage considered in this context is the twisty flow pattern emerging within the droplets when they are moved through the channel system. Mixing of two aqueous components is achieved in this way within each droplet separately, and is found to proceed quite efficiently.

Another advantage of this approach, which has been rarely referred to (if at all), is that each droplet may carry chemically different contents. If the volume fraction of the oily phase is kept small, the droplets will touch each other, separated only by a thin oil lamella. A chemical reaction may then be induced, at a precisely defined time, by destroying the lamella, e.g., by local heating.

Discrete flow (droplet-based)

Electrowetting actuation

Electrowetting has recently been used successfully as one of several techniques used to actuate microdroplets in a digital microfluidic device. In many of these applications electowetting allows large numbers of droplets to be independently manipulated under direct electrical control without the use of pumps, valves or even fixed channels. The phenomenon of electowetting can be understood in terms of the forces that result from the applied electric field. The fringing field at the corners of the electrolyte droplet tend to pull the droplet down onto the electrode, lowering the macroscopic contact angle, and increasing the droplet contact area. (Wikipedia article)

ElectroWetting-On-Dielectric (EWOD), which is based on changing the wettability of liquids on a dielectric solid surface by varying the electric potential. This method offers advantages over conventional continuious-flow microfluidic chips, by way of significantly reduced sample size, as well as reconfigurability and scalability of architecture. The similarity of the EWOD system with digital microelectronic systems, has led to the term “digital microfluidics” [1]. The phenomenon of EWOD has been demonstrated for dispensing, cutting, and transport of tiny droplets [2, 3], and more recently, a proof-of-concept has been demonstrated for an integrated lab-on-a-chip system for clinical diagnostic applications